https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70255
Bug ID: 70255
Summary: change of the order of summation of floating point
numbers despite no-associative-math
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: shatz at dsit dot co.il
Target Milestone: ---
Created attachment 37989
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37989&action=edit
minimal test case of change of the order of summation
Under certain very specific combination of factor gcc optimizer disregards
order of summation specified in the source code despite "no-associative-math"
option specified by means of function attribute.
The bug does not happen when -fno-associative-math option specified on the
command line.
In the file, attached below there are 3 very similar versions of the function
foo(). Only variant foo1() shows the bug.
Here is a asm code generated for foo1():
foo1:
.seh_endprologue
vmovsd .LC0(%rip), %xmm2
vaddsd %xmm2, %xmm0, %xmm3
vsubsd %xmm3, %xmm0, %xmm0
vaddsd %xmm1, %xmm0, %xmm1 ; order of summation changed!!!!
vaddsd %xmm2, %xmm1, %xmm0
ret
.seh_endproc
Here is an output of gcc -v:
Using built-in specs.
COLLECT_GCC=D:\bin\msys64\mingw64\bin\gcc.exe
Target: x86_64-w64-mingw32
Configured with: ../gcc-5.2.0/configure --prefix=/mingw64
--with-local-prefix=/mingw64/local --build=x86_64-w64-mingw32
--host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32
--with-native-system-header-dir=/mingw64/x86_64-w64-mingw32/include
--libexecdir=/mingw64/lib --with-gxx-include-dir=/mingw64/include/c++/5.2.0
--enable-bootstrap --with-arch=x86-64 --with-tune=generic
--enable-languages=c,lto,c++,objc,obj-c++,fortran,ada --enable-shared
--enable-static --enable-libatomic --enable-threads=posix --enable-graphite
--enable-fully-dynamic-string --enable-libstdcxx-time=yes
--disable-libstdcxx-pch --disable-libstdcxx-debug
--enable-version-specific-runtime-libs --disable-isl-version-check --enable-lto
--enable-libgomp --disable-multilib --enable-checking=release --disable-rpath
--disable-win32-registry --disable-nls --disable-werror --disable-symvers
--with-libiconv --with-system-zlib --with-gmp=/mingw64 --with-mpfr=/mingw64
--with-mpc=/mingw64 --with-isl=/mingw64 --with-pkgversion='Rev4, Built by MSYS2
project' --with-bugurl=http://sourceforge.net/projects/msys2 --with-gnu-as
--with-gnu-ld
Thread model: posix
gcc version 5.2.0 (Rev4, Built by MSYS2 project)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-Ofast' '-Wall' '-mavx'
'-mtune=generic' '-march=x86-64'
D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/cc1.exe -E
-quiet -v -iprefix
D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/ -D_REENTRANT
foo.c -mavx -mtune=generic -march=x86-64 -Wall -Ofast -fpch-preprocess -o foo.i
ignoring duplicate directory
"D:/bin/msys64/mingw64/lib/gcc/../../lib/gcc/x86_64-w64-mingw32/5.2.0/include"
ignoring nonexistent directory "C:/building/msys64/mingw64/include"
ignoring nonexistent directory "/mingw64/include"
ignoring duplicate directory
"D:/bin/msys64/mingw64/lib/gcc/../../lib/gcc/x86_64-w64-mingw32/5.2.0/include-fixed"
ignoring duplicate directory
"D:/bin/msys64/mingw64/lib/gcc/../../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../../x86_64-w64-mingw32/include"
ignoring nonexistent directory
"C:/building/msys64/mingw64/x86_64-w64-mingw32/include"
#include "..." search starts here:
#include <...> search starts here:
D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/include
D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../../include
D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/include-fixed
D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../../x86_64-w64-mingw32/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-Ofast' '-Wall' '-mavx'
'-mtune=generic' '-march=x86-64'
D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/cc1.exe
-fpreprocessed foo.i -quiet -dumpbase foo.c -mavx -mtune=generic -march=x86-64
-auxbase foo -Ofast -Wall -version -o foo.s
GNU C11 (Rev4, Built by MSYS2 project) version 5.2.0 (x86_64-w64-mingw32)
compiled by GNU C version 5.2.0, GMP version 6.0.0, MPFR version 3.1.3,
MPC version 1.0.3
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C11 (Rev4, Built by MSYS2 project) version 5.2.0 (x86_64-w64-mingw32)
compiled by GNU C version 5.2.0, GMP version 6.0.0, MPFR version 3.1.3,
MPC version 1.0.3
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 6d1a96b0d4d4a024cf53bcf4b7004358
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-Ofast' '-Wall' '-mavx'
'-mtune=generic' '-march=x86-64'
D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../../x86_64-w64-mingw32/bin/as.exe
-v -o foo.o foo.s
GNU assembler version 2.25.1 (x86_64-w64-mingw32) using BFD version (GNU
Binutils) 2.25.1
COMPILER_PATH=D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/;D:/bin/msys64/mingw64/bin/../lib/gcc/;D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../../x86_64-w64-mingw32/bin/
LIBRARY_PATH=D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/;D:/bin/msys64/mingw64/bin/../lib/gcc/;D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../../x86_64-w64-mingw32/lib/../lib/;D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../../lib/;D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../../x86_64-w64-mingw32/lib/;D:/bin/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/5.2.0/../../../
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-Ofast' '-Wall' '-mavx'
'-mtune=generic' '-march=x86-64'
